The present invention provides an apparatus and method for determining the magnitude of linear and rotational acceleration of an impact to a body part. The apparatus can be used with protective sports equipment, such as a sports helmet, wherein the apparatus includes a battery, a number of accelerometers positioned proximate to the outer surface of the head, and an electronic device with a processor and a transmitter to transmit data received from the accelerometers. To maximize the battery life and minimize power consumption by the electronic device, the apparatus includes a power management system with a sensor assembly. The sensor assembly sends a first signal to the electronic device to initiate operation when the sensor assembly detects the presence of an object within the helmet, and a second signal to the electronic device to cease operation when the sensor assembly detects the absence of the object. The sensor assembly may be a proximity sensor, more specifically an inductive, capacitive, or ultrasonic sensor.